Peirce on Abduction

The term “abduction” was coined by Charles Sanders Peirce
in his work on the logic of science. He introduced it to denote a
type of non-deductive inference that was different from the already
familiar inductive type. It is a common complaint that no coherent
picture emerges from Peirce's writings on abduction. (Though
perhaps this is not surprising, given that he worked on abduction
throughout his career, which spanned a period of more than fifty
years. For a concise yet thorough account of the development of
Peirce's thoughts about abduction, see Fann 1970.) Yet it is
clear that, as Peirce understood the term, “abduction”
did not quite mean what it is currently taken to mean. One main
difference between his conception and the modern one is that, whereas
according to the latter, abduction belongs to what the logical
empiricists called the “context of
justification”—the stage of scientific inquiry in which
we are concerned with the assessment of theories—for Peirce
abduction had its proper place in the context of discovery, the stage
of inquiry in which we try to generate theories which may then later
be assessed. As he says, “[a]bduction is the process of forming
explanatory hypotheses. It is the only logical operation which
introduces any new idea” (CP 5.172); elsewhere he says that
abduction encompasses “all the operations by which theories and
conceptions are engendered” (CP 5.590). Deduction and
induction, then, come into play at the later stage of theory
assessment: deduction helps to derive testable consequences from the
explanatory hypotheses that abduction has helped us to conceive, and
induction finally helps us to reach a verdict on the hypotheses,
where the nature of the verdict is dependent on the number of
testable consequences that have been verified. (As an aside, it is to
be noted that Gerhard Schurz has recently defended a view of
abduction that is again very much in the Peircean spirit. On this
view, “the crucial function of a pattern of abduction …
consists in its function as a search strategy which leads us, for a
given kind of scenario, in a reasonable time to a most promising
explanatory conjecture which is then subject to further test”
(Schurz 2008, 205). The paper is also of interest because of the
useful typology of patterns of abduction that it puts forth.)

As Harry Frankfurt (1958) has noted, however, the foregoing view is
not as easy to make sense of as might at first appear. Abduction is
supposed to be part of the logic of science, but what exactly is
logical about inventing explanatory hypotheses? According to
Peirce (CP 5.189), abduction belongs to logic because it can be given
a schematic characterization, to wit, the following:

The surprising fact, C, is observed.
But if A were true, C would be a matter of course.
Hence, there is reason to suspect that A is true.

But Frankfurt rightly remarks that this is not an inference leading
to any new idea. After all, the new idea—the explanatory
hypothesis A—must have occurred to one before
one infers that there is reason to suspect that A is
true, for A already figures in the second premise.

Frankfurt then goes on to argue that a number of passages in
Peirce's work suggest an understanding of abduction not so much
as a process of inventing hypotheses but rather as one of
adopting hypotheses, where the adoption of the hypothesis is
not as being true or verified or confirmed, but as being a worthy
candidate for further investigation. On this understanding, abduction
could still be thought of as being part of the context of discovery. It would
work as a kind of selection function, or filter, determining which of
the hypotheses that have been conceived in the stage of discovery are
to pass to the next stage and be subjected to empirical testing. The
selection criterion is that there must be a reason to suspect that
the hypothesis is true, and we will have such a reason if the
hypothesis makes whichever observed facts we are interested in
explaining a matter of course. This would indeed make better sense of
Peirce's claim that abduction is a logical operation.

Nevertheless, Frankfurt ultimately rejects this proposal as well.
Given, he says, that there may be infinitely many hypotheses that
account for a given fact or set of facts—which Peirce
acknowledged—it can hardly be a sufficient condition for the
adoption of a hypothesis (in the above sense) that its truth would
make that fact or set of facts a matter of course. At a minimum,
abduction would not seem to be of much use as a selection function.
One may doubt whether this is a valid objection, however. Echoing
what was said in connection with underdetermination arguments, we
note that it is by no means clear that “accounting for a given
fact” is to be identified with “making that fact a matter
of course.” For all Frankfurt says, for a hypothesis to account
for a fact, it is enough if it entails that fact. But virtually no
philosopher of science nowadays holds that entailment is sufficient
for explanation. And it would seem reasonable to read the phrase
“making a given fact a matter of course” as “giving
a satisfactory explanation of that fact.” In response to
Frankfurt's objection, it could thus be argued that even if
there is an infinity of hypotheses that account for a given fact,
there may still be only a handful that could be said to give a
satisfactory explanation of it. But it is for Peirce scholars to
decide whether this proposed interpretation is plausible in the light
of Peirce's further writings.

Even if “making a given fact a matter of course” can be
read as “giving a satisfactory explanation of that fact,”
it is remarkable that there is no reference in Peirce's
writings on abduction to the notion of best explanation.
Some satisfactory explanations might still be better than others, and
there might even be a unique best one. This idea is crucial in all
recent thinking about abduction. Therein lies another main difference
between Peirce's conception of abduction and the modern one.